In many applications, a common signal path is coupled both to the input of a receiver and to the output of a transmitter. For example, in a transceiver, such as a cellular or cordless telephone, an antenna may be coupled to the input of the receiver and to the output of the transmitter. In such an arrangement, a duplexer is used to couple the common signal path to the input of the receiver and to the output of the transmitter. The duplexer provides the necessary coupling while preventing the modulated transmit signal generated by the transmitter from being coupled from the antenna back to the input of the receiver and overloading the receiver.
Often, among other elements, resonators are used to prevent the undesired coupling of these signals. One type of filter is based on a film bulk acoustic resonator (FBAR) structure. The FBAR includes an acoustic stack comprising a layer of piezoelectric material disposed between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack.
FBARs are similar in principle to bulk acoustic resonators such as quartz, but are scaled down to resonate at GHz frequencies. Because the FBARs have thicknesses on the order of microns, and length and width dimensions of hundreds of microns, FBARs beneficially provide a comparatively compact alternative to known resonators.
More and more there is a need for differential signal applications from a single ended input. This has led to the investigation of alternative filter arrangements.
One way of providing a single-ended to differential signal transformation in a filter application involves a device known as a balun. For example, the balun may be connected to an FBAR-based filter. Unfortunately, and among other drawbacks, the use of a balun adds another (external) element to circuit, driving the cost and size of the filter up.
One known resonator structure having a differential output comprises coupled mode resonators. Coupled mode resonators often comprise an upper FBAR and a lower FBAR, with a layer of acoustic decoupling material between the two FBARs. The two electrodes of one of the FBARs comprise the differential outputs, and one of the inputs to the lower resonator provides the single-ended input. The second electrode provides the ground for the device.
While the noted structure is beneficial in providing a single-ended to differential signal transformation device, a parasitic capacitance is formed by the ground electrode, the decoupling layer and one of the differential output electrodes. The net result is an unacceptable amplitude imbalance, or phase imbalance, or both between the differential outputs. Thus, the parasitic capacitance, as well as other parasitic components that can be external to the differential output resonator, can have a deleterious impact on the function of the differential resonator.
There is a need, therefore, for a single-ended input to differential output filter that overcomes at least the shortcoming of known filters discussed above.